There is a great need in many applications to monitor the positions of a movable object relative to a desired position or positions. For example, it is desirable in disk drive servo systems to accurately detect the positions of a rotatable read/write head arm, relative to desired, known positions, so that the head arm can be moved into alignment with desired radial track locations on a disk to enable the read/write head to read from and write to such locations. Position sensing devices are devices aimed at providing such position detection. Displacement sensors are position sensing devices that monitor the positions of a movable object by repeatedly measuring the displacements of the object from a desired location.
Examples of conventional displacement sensors include capacitance gage devices, fiberoptic proximity sensors and optical sensors, such as interferometric sensors used commonly in disk drive servo systems. Many of the prior art displacement sensors either are incapable of achieving the degree of resolution (the smallest measureable displacement) required by certain applications (such as disk drive servo systems) or require expensive and/or complex circuitry and hardware to achieve such resolution. To achieve high resolution, certain displacement sensors require, for example, a precise and powerful laser light source and extremely close proximity between the sensing element and movable object, rendering such devices expensive and difficult to implement. Thus, a balance must be struck between device performance and simplicity.
U.S. Pat. No. 5,315,372 to Tsai describes a prior art disk drive servo system that employs an optical displacement sensor. The Tsai device includes a light source and a photodetector array attached, at spaced-apart locations, to a rotatable master arm, located externally of the disk drive. A reflector is attached to a rotatable read/write head arm at a location between the read/write head and the axis of rotation of the arm. During operation, the master arm first is aligned accurately with a desired radial track location on the disk using an interferometric device. Then, the displacement sensor operates to determine the position of the head arm relative to the master arm so that the head arm can be moved into alignment with the master arm.
The light source of the displacement sensor produces an incident light beam that is reflected by the reflector onto the photodetector array. The position on the photodetector array to which the light beam is reflected depends on the relative radial positions of the head and master arms. Each photodetector element of the array produces an electric signal having an amplitude proportional to the intensity of the received light. The signals produced by the array, thus, represent the relative radial positions of the head and master arms. Processing circuitry receives and decodes the signals output by the array to determine the relative head arm position and accordingly controls a motor to rotate the head arm until it is properly aligned.
The device disclosed in the Tsai patent suffers from a number of drawbacks. While the Tsai device is relatively structurally simple and fairly inexpensive to implement, it is burdensome to operate. The Tsai device requires a pre-knowledge of each master arm and relative head arm position in order to accurately interpret the electric signals produced by the photodetector array. Also, due to the significant spacing between the reflector and the read/write head on the head arm and, due also to the dual axes of rotation of the head arm and master arm, performance accuracy is sacrificed severely.
It accordingly is a general object of the present invention to provide a simple, yet accurate, displacement sensor.